This invention relates to electro-optical devices whose electromagnetic radiation absorption characteristics can be selectively altered by influence of a suitably controlled electric field. More particularly, the invention is concerned with an electro-optical device which contains a layer of electrochromic material, having a greatly simplified and more effective counter-electrode. Still more particularly, this invention is directed to a sandwich type cell in which two layers of electrochromic material are separated by solid, semi-solid or liquid ion conducting media. It particularly relates to a greatly simplified method for forming counter-electrodes and electrochromic layers.
In prior U.S. applications, Ser. No. 530,086 filed Feb. 25, 1966, now abandoned, refiled as Ser. No. 616,791 filed Feb. 14, 1967, now abandoned, refiled as Ser. No. 110,068 filed Jan. 27, 1971, now abandoned, refiled as Ser. No. 349,878 filed Apr. 10, 1973, now U.S. Pat. No. 3,829,196, and Ser. No. 534,188 filed Mar. 14, 1966, now abandoned, refiled as Ser. No. 616,790 filed Feb. 17, 1967, now abandoned refiled as Ser. No. 120,365 filed Mar. 2, 1971, now abandoned, and Ser. No. 41,153 filed May 25, 1970, now abandoned, refiled as Ser. No. 211,857 filed on Dec. 23, 1971, now abandoned, refiled as Ser. No. 361,760 filed on May 18, 1973, now U.S. Pat. No. 3,879,108 there are described electro-optical devices exhibiting a phenomenon known as persistent electrochromism wherein electromagnetic radiation absorption characteristic of a persistent electrochromic material is altered under the influence of an electric field. Such devices were employed in sandwich arrangement between two electrodes. Coloration was induced by charging the electrochromic film negative with respect to the counter-electrode, employing an external potential. The counter-electrode can be the same as the persistent electrochromic material or different.
By reversing the original polarity of the field or by applying a new field, it was also possible to cancel, erase or bleach the visible coloration.
These steps of color induction and erasure are defined as cycling.
Although the devices described in the prior applications are effective to change their electromagnetic radiation transmitting properties under the influence of an electric field, the practicality of a simple sandwiched or layered arrangement of electrodes and layer of electrochromic material is somewhat limited due to the complexity in formation or configuration of counter-electrodes. Moreover, prior counter-electrodes did not exhibit high speed switching capability or extended cycle lifetime. In addition, their fabrication was often complicated. Electrochromic layers have previously been vacuum deposited. Counter-electrodes have been formed from vacuum deposited metal layers and also as compacted mixtures of graphite, fibrillated teflon, and an electrochromic material.
This shortcoming has been overcome in the present invention by employing a counter-electrode which is deposited as a layer from a simple solution or suspension, under ambient conditions.